Our laboratory has previously shown that aging affects myocardial function in rats, specifically that parameters measuring the duration of the isometric twitch increase with age. It has also been shown that myofibrillar ATPase activity as well as responsiveness of vascular smooth muscle (VSM) to adrenergic agents decreases as the animal ages. Short term physical conditioning reversed the changes in myocardial performance seen in the senescent heart in rats. Because of the emphasis of exerrcise in relationship to health, we designed a chronic exercise model in the rat to evaluate the relationship between exercise and aging. A swimmming model has been developed in which daily swimming is begun in rats 5 weeks of age and continued for the duration of their lives. We have been able to obtain adequate survival to 18 months of age with isolated survivors beyond that time. Mortalities, commonly during swimming, have been analyzed as to the effect of chronic exercise on body weight and heart size. We have recently started to sacrifice the animals across a wide age range to evaluate the effect of lifelong exercise on myocardial and VSM function as well as myofibrillar ATPase activity.